1. Technical Field
The present invention relates to a radar apparatus, and more particularly to a radar apparatus provided with series-feed array-antennas each including a plurality of antenna elements series-connected by feedlines.
2. Description of the Related Art
In a radar apparatus using an array-antenna, the feeding phase of the antenna elements is shifted due to occurrence of an error when the radar apparatus is manufactured or due to a variation of the ambient temperature. Hence, direction of the radar beam outputted from the array-antenna may be shifted as well. As a result, characteristics of the radar apparatus may be degraded.
In this regard, for example, Japanese Patent Application Laid-Open Publication Nos. 2008-224511 and 2002-162460 disclose techniques used in radar apparatuses such that the phase shift of the radar apparatus is calibrated.
Specifically, the patent document No. 2008-224511 discloses a technique in which leaked waves from the transmission antenna is received by the reception antenna and the phase of the leak waves is stored, and the method compares a phase of radar waves when an object is actually detected with the phase of the leak waves, then extracts an amount of phase-correction for each antenna element and memorize the amount of phase-correction. As a result, the error is calibrated based on the amount of phase-correction every time when the object is detected.
Moreover, the patent document No. 2002-162460 discloses a technique in which a phase difference between the radar waves at a reference antenna and the radar waves at other antennas is detected, and comparing the detected phase difference with estimated phase difference due to temperature variation at other antenna elements, the amount of phase-correction is obtained.
According to a millimeter-wave radar apparatus, to decrease a power loss of the feedline, length of the feedline can be shortened. Hence, it is effective to use a series-feed array-antenna in the millimeter-wave radar apparatus. However, in the series-feed array-antenna, due to occurrence of an error when the series-feed array-antenna is manufactured or due to a temperature variation, an error may occur in the phase of feed (feeding phase) for the respective antenna elements so that a peak angle of directivity is shifted. As a result, performance of the radar apparatus may be degraded. For instance, as shown in FIG. 5, the beam of the radar waves is designed to have a peak of the gain at 0 deg when no error is expected in the radar apparatus. Assuming an error occurs in the radar apparatus so that the peak angle of the directivity is shifted by θ deg, the gain at the 0 deg direction is significantly decreased.
The techniques disclosed by the patent documents Nos. 2008-224511 and 2002-162460 are related to a calibration of an angle-error in the horizontal direction of the radar. Therefore, the correction cannot be made in the vertical direction of the radar. This is because a phase shift of the feeding in respective antenna elements arrayed in the vertical direction cannot be detected and corrected. Also, in the technique disclosed by the patent document 2002-162460, it is necessary to set the reference antenna element and to dispose a standard reflection body in front of the array-antenna in order to detect the initial phase. Therefore, in this technique, the calibration of the antennas cannot be made easily.